grandpa: missing equivocation reporting nits (#5953)

* primitives: move reporting key type to common key types

* session: remove useless methods on MembershipProof

* grandpa: remove std special-casing when checking signatures

* grandpa: add some more docs

* grandpa: use proper error types rather than strings

substrate/frame/grandpa/src/equivocation.rs

@@ -29,6 +29,12 @@
 //! And in a runtime context, so that the GRANDPA module can validate the
 //! equivocation proofs in the extrinsic and report the offences.
 //!
+//! IMPORTANT:
+//! When using this module for enabling equivocation reporting it is required
+//! that the `ValidateEquivocationReport` signed extension is used in the runtime
+//! definition. Failure to do so will allow invalid equivocation reports to be
+//! accepted by the runtime.
+//!
 
 use sp_std::prelude::*;
 
@@ -395,12 +401,12 @@ impl GetValidatorCount for frame_support::Void {
 
 impl GetSessionNumber for sp_session::MembershipProof {
 	fn session(&self) -> SessionIndex {
-		self.session()
+		self.session
 	}
 }
 
 impl GetValidatorCount for sp_session::MembershipProof {
 	fn validator_count(&self) -> sp_session::ValidatorCount {
-		self.validator_count()
+		self.validator_count
 	}
 }